1. Field of the Invention
The present invention generally relates to a tank venting valve for relieving the internal pressure in tanks at a predetermined pressure. More particularly the invention relates to a pilot operated tank vent valve.
2. Related Art
There are several types of pressure relief valves for atmospheric or low pressure storage tanks. One of the basic designs is known as a "weight loaded tank vent". The design utilizes a weighted cover over a nozzle. The cover, also known as a pallet, forms a barrier to the nozzle opening. The weight of the pallet determines at what pressure the pallet will lift open to relieve pressure in the tank. The opening pressure is known as the "set pressure". The set pressure can easily be determined by dividing the weight of the pallet assembly by the open area of the nozzle. If a higher set pressure is desired more weight is added to the pallet.
Another type of pressure relieving device is a spring loaded tank vent. The spring loaded type is used for set pressures above that which is practical for weight loaded operation due to either a high set pressure where there is not enough room for the weights on the pallet, or where the size of the vent nozzle dictates a very heavy weight on the pallet.
A third type of tank vent is pilot operated. Pilot operated tank vents utilize the tank pressure acting on an area larger than the nozzle area to create a higher downward force to hold down a pallet or seat plate against the tank pressure. For this reason pilot operated tank vents seal tighter than weight or spring loaded vents until the set or opening pressure is reached. The set pressure is adjustable by means of a spring in the pilot.
Any of the three types may be vented to the atmosphere or piped into headers for vapor recovery or burning.
It is a particular feature of the present vent valve that it is adapted for use with a pilot valve. It is an advantage of the present vent valve that the main valve automatically prevents back flow in the event the outlet header pressure is higher than the tank pressure. A further advantage of the present vent valve is that the main valve will open substantially at the set pressure even with varying pressure in the outlet header, whether the header pressure is positive (above zero gauge) or negative (below zero gauge). It is a further advantage that the present vent valve is modular in the components of the valve, actuator and associated pilot valve. These and other features and advantages will become apparent from the following descriptions.